This study examined the time course of the aerobic response to strenuous training in well-conditioned distance runners when intensity was held fairly constant relative to maximal aerobic power, and the effect of this training on 10-km run time. Seven runners trained 6 d/wk for 6 wk. On 3 d/wk they ran as far as possible in 40 min, with the intention of running a greater distance each run. On alternate days the subjects performed five 5-min rides on a cycle ergometer (CE) separated by 5-min intervals of jogging, with the resistance adjusted to induce peak-CE VO2 during minutes 4 and 5 of each ride. Peak-CE VO2 increased (56.1 to 65.0 ml X min-1 X kg-1, P less than 0.05) in a linear manner (r = 0.95, P less than 0.01) during the first 5 wk. The average weekly increase was 0.11 l X min-1. Unexpectedly, peak-CE VO2 decreased significantly after week six. Treadmill VO2max was not significantly different after training. The mean decrease (P less than 0.05) in 10-km run time was approximately 81 S. These data suggest an adaptation of constant magnitude of the factor(s) which limit peak-CE VO2 when training intensity is maintained relative to improvement. However, this linear response and training intensity may be maintained only on a limited basis without expecting some decline in performance and aerobic power.